The pavement marking industry has long desired transparent, solid microspheres or beads that would be useful as brighter and more durable retroreflective lens elements in pavement markings. The transparent microspheres now most widely used for pavement markings are made of certain glasses, which are amorphous vitreous materials. Generally these glasses are of the soda-lime-silicate type having a refractive index of only about 1.5, which limits their retroreflective brightness. Glass microspheres of improved durability and a higher refractive index have been taught in U.S. Pat. No. 4,367,919.
A transparent ceramic microsphere made by a sol-gel process from silica and zirconium compounds is taught in U.S. Pat. No. 3,709,706. Generally, a sol-gel process is one which converts a colloidal dispersion, sol, aquasol or hydrosol of a metal oxide (or precursor thereof) to a gel. A gel is a material form wherein one or more of the components are crosslinked either chemically or physically to such an extent as to cause a three dimensional network to form. The formation of this network results in an increase in viscosity of the mixture and a mechanical immobilization of the liquid phase within the network. The gelling step is often followed by drying and then firing to obtain a ceramic material.
Microspheres used in pavement markings generally average between about 100 and 1000 micrometers in diameter in order to assure that the light-gathering portion of the microspheres protruding from the pavement marking is not obscured by road dirt.
The transparent microspheres used in retroreflective sheeting applications such as pavement marking sheets typically have an index of refraction between about 1.5 and 2.5. A refractive index (N.sub.D) of at least 1.7 provides good reflectivity under dry conditions (N.sub.D of 1.9 being preferred), and some or all of the microspheres should have an index of refraction of at least 2.2 if wet reflection is desired.
In zirconia-silica (ZrO.sub.2 -SiO.sub.2) ceramic microspheres, which are known to the art, zirconia is believed to be the constituent which imparts toughness, durability, strength and high refractive index. However, increasing the mole ratio of ZrO.sub.2 to SiO.sub.2 much above 1.3:1 can cause increased cracking of the microspheres during processing.